I18N Improvements in 4.6

WordPress 4.6 significantly improves the way translations are loaded for plugins and themes which are hosted in the WordPress Plugin and Theme Directory and using language packs via WordPress.org’s translation platform.

Loading Translations in Different Order

Functions like load_plugin_textdomain( $domain ) and load_theme_textdomain( $domain ) are used to load the translations for a specific text domain, where $domain equals the slug of your plugin or theme.

Up until now, these functions first looked inside the plugin/theme folder for available translations before checking the wp-content/languages directory, where language packs reside. With WordPress 4.6, this order has been reversed.
The translation platform for plugins/themes was opened up in December last year. Since then 15,000 plugins and themes were imported and are now benefiting from automated translation updates. That’s why translation files are most likely to be located wp-content/languages directory, hence this change. See [37414] / #34213.

Just-in-time Loading for Translations

By far the biggest change is that you do not have to call load_plugin_textdomain() or load_theme_textdomain() anymore with WordPress 4.6. Of course the same goes for load_muplugin_textdomain().

Again, since translations files are usually inside wp-content/languages, WordPress now scans that directory for available translations and automatically loads them if it encounters a text domain for the first time. That’s handled by the new _load_textdomain_just_in_time() function which gets called in get_translations_for_domain(). See [37415] / #34114.

This so-called just-in-time loading of text domains works really great and removes one more step for developers to worry about. If you do not want this to happen, however, you can still use the override_load_textdomain filter to load your text domains manually.

Also, if you use unload_textdomain() you will have to manually load translations afterwards if you want to use them again because it keeps a record of unloaded text domains.

This is probably best asked in the support forums, but here’s a quick answer: Create a function that hooks into `override_load_textdomain` and manually loads translations from another location. If that is successful, it returns `true` to indicate the overloading was successful.

Here are some examples I found from quick Google searches to help get you started:

I’m having the same question as @chiara. I gave the `override_load_textdomain` filter a try, but no success so far. The links Pascal is pointing towards can give us a direction, but they don’t provide a solution that works out-of-the-box with WP 4.6.